1. Field of the Invention
This invention relates to thermal ink jet printing, and more particularly to large array thermal ink jet printheads and fabricating process therefor.
2. Description of the Prior Art
Thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printing system may be incorporated in either a carriage type printer or a pagewidth type printer. The carriage type printer generally has a relatively small printhead, containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. For an example of a cartridge type printer, refer to U.S. Pat. No. 4,571,599 to Rezanka. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process. Refer to U.S. Pat. No. 4,463,359 to Ayata et al for an example of pagewidth printing and especially FIGS. 17 and 20 therein.
U.S. Pat. No. 4,463,359 mentioned above discloses a printhead having one or more ink filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and brake off into a droplet at the beginning of the bubble collapse. The current pulses are shaped to prevent the meniscus from breaking up and recording too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are shown, such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate for the purpose of obtaining a pagewidth printhead. Such arrangements may also be used for different colored inks to enable multi-colored printing.
U.S. Pat. Re. No. 33.572 to Hawkins et al discloses a thermal ink jet printhead and method of fabrication. In this case, a plurality of printheads may be concurrently fabricated by forming a plurality of sets of heating elements with their individual addressing electrodes on one substrate and etching corresponding sets of channel grooves with a common recess for each set of grooves in a wafer. The wafer and substrate are aligned and bonded together so that each channel has a heating element. The individual printheads are obtained by milling away the unwanted silicon material to expose the addressing electrode terminals and then dicing the substrate to form separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al discloses an improved printhead of the type disclosed in the patent to Hawkins et al wherein the bubble generating resistors are located in recess to prevent lateral movements of the bubbles through the nozzles and thus preventing sudden release of vaporized ink to the atmosphere.
U.S. Pat. No. 4,639,748 to Drake et al discloses another improvement in the printhead of the type disclosed in the patent to Hawkins et al. In this patent, the common manifold for the ink channels contains an integral filter which prevents contaminates in the ink from reaching the printhead nozzles.
U.S. Pat. No. 4,678,529 to Drake et al discloses a method of bonding the ink jet printhead channel plate and heater plates together by a process which provides the desired uniform thickness of adhesive on the mating surfaces and preventing the flow of adhesive into the fluid passageways.
U.S. Pat. No. 4,612,554 to Poleshuk discloses an ink jet printhead composed of two identical parts, each having a set of parallel V-grooves anisotropically etched therein. The lands between the grooves each contain a heating element and its associated addressing electrodes. The grooved parts permit face-to-face mating, so that they are automatically self-aligned by the intermeshing of the lands containing the heating elements and electrodes of one part with the grooves of the other parts. A pagewidth printhead is produced by offsetting the first two mated parts, so that subsequently added parts abut each other and yet continue to be self-aligned.
A copending and commonly assigned U.S. patent application, Ser. No. 082,417, filed Aug. 6, 1987, entitled "Thermal Ink Jet Printhead and Fabricating Process Therefor" to Drake et al, discloses a thermal ink jet printhead of the type which expels droplets on demand towards a recording medium from nozzles located above and generally parallel with the bubble generating heating elements contained therein. The droplets are propelled from nozzles located in the printhead roof along trajectories that are perpendicular to the heating element surfaces. Such configurations is sometimes referred to as "roofshooter". Each printhead comprises a silicon heater plate and a fluid directing structural member. The heater plate has a linear array of heating elements, associated addressing elements, and an elongated ink fill hole parallel to and adjacent the heating element array. A structural member contains at least one recessed cavity, a plurality of nozzles, and a plurality of parallel walls within the recessed cavity which define individual ink channels for directing ink to the nozzles. The recessed cavity and fill hole are in communication with each other and form the ink reservoir within the printhead. The ink holding capacity of the fill hole is larger than that of the recessed cavity. The fill hole is precisely formed and positioned within the heater plate by anisotropic etching. The structural member may be fabricated either from two layers of photoresist, a two-stage flat nickel electroform, or a single photoresist layer and a single stage flat nickel electroform.
Copending and commonly assigned U.S. patent application Ser. No. 115,271 filed Nov. 2, 1987, entitled "An Improved Ink Jet Printhead" to Hawkins, now U.S. Pat. No. 4,774,530, discloses the use of an etched thick film insulative layer to provide the flow path between the ink channels and the manifold, and copending and commonly assigned U.S. patent application Ser. No. 126,085, filed Nov. 27, 1987, entitled "Thermal Ink Jet Printhead and Fabrication Method Therefor" to Campanelli et al, no U.S. Pat. No. 4,786,352 discloses the use of an etched thick film insulative layer between mated and bonded substrates. One substrate has a plurality of heating element arrays and addressing electrodes formed on the surface thereof and the other being a silicon wafer having a plurality of etched manifolds, with each manifold having a set of ink channels. The etched thick film layer provides a clearance space above each set of contact pads of the addressing electrodes to enable the removal of the unwanted silicon material of the wafer by dicing without the need for etched recesses therein. The individual printheads are produced subsequently by dicing the substrate having the heating element arrays.
Drop-on-demand thermal ink jet printheads discussed in the above patents are fabricated by using silicon wafers and processing technology to make multiple small heater plates and channel plates. This works extremely well for small printheads. However, for large array or pagewidth printheads, a monolithic array of ink channels cannot be practically fabricated in a single wafer since the maximum commercial wafer size is six inches. Even if ten inch wafers were commercially available, it is not clear that a monolithic channel array would be very feasible. This is because only defective channel out of 2,550 channels would render the entire channel plate useless. This yield problem is aggravated by the fact that the larger the silicon ingot diameter, the more difficult it is to make it defect-free. Also, relatively frew 81/2 inch channel plate arrays could be fabricated in a ten inch wafer. Most of the wafer would be thrown away, resulting in very high fabrication costs.
The fabrication approaches for making either large array or pagewidth therml ink jet printheads can be divided into basically two broad categories; namely, monolithi approaches in which one or both of the printhead components (heater substrate and channel plate substrate) are a single large array or pagewidth size, or sub-unit approaches in which smaller sub-units are combined to form the large array or pagewidth print bar. For an example of the sub-unit approach, refer to the abovementioned U.S. Pat. No. 4,612,554 to Poleshuk, and in particular to FIG. 7 thereof. The sub-units approach may give a much higher yield of usable sub-units, if they can be precisely aligned with respect to each other. The assembly of a plurality of sub-units, however, require precise individual registration in both the x-y-z planes as well as the angular registration within these planes. The alignment problems for these separate units presents quite a formidable task, the prior art solution of which makes this type of large array very expensive.